1. Field of the Invention
The present invention relates to a scanning type exposure apparatus and a device manufacturing method and, more particularly, to a scanning type exposure apparatus and a device manufacturing method suitably applied when various devices, such as a semiconductor device such as an IC or an LSI, a display device such as a liquid crystal display element, and a sensor device such as a magnetic head, are manufactured.
2. Description of the Related Art
In recent years, the integration density of a semiconductor device such as an IC or an LSI has increasingly accelerated, and a micropatterning technique for a semiconductor wafer in accordance with the increase in integration density has considerably developed. As examples of the micropatterning technique, various reduced projection exposure apparatuses (steppers), each of which forms the image of a circuit pattern on a mask (reticle) onto a photosensitive substrate by a projection optical system and exposes the photosensitive substrate by a step and repeat scheme, have been proposed.
In this type of stepper, a circuit pattern on a reticle is reduced and projected at a predetermined position (shot) onto a wafer surface through a projection optical system having a predetermined reduction ratio to be transferred, and, after the projection and transformation are performed once, the stage on which the wafer is placed is moved by a predetermined amount to transfer the image on another shot. These steps are repeated to entirely expose the wafer. In recent years, as examples of such projection exposure apparatuses, various step and scan exposure apparatuses, each using a scanning mechanism which can obtain a high resolution and extend the size of the exposure region, have been proposed.
Such a step and scan exposure apparatus has a slit-like exposure area and scans a reticle and a wafer with respect to an illumination optical system and a projection optical system to expose a shot. Upon completion of scanning exposure of one shot, the wafer shifts to the next step, and scanning exposure of the next shot is performed. This procedure is repeated to entirely expose the wafer.
FIG. 10 is a perspective view showing a main portion of a conventional step and scan exposure apparatus. Referring to FIG. 10, reference numeral 201 denotes a reticle on which a circuit pattern is drawn, and reference numeral 202 denotes a projection lens for projecting the circuit pattern on the reticle 201 onto a semiconductor wafer 203. Reference numeral 204 denotes a stage on which the semiconductor wafer 203 is placed.
An illumination optical system 205 does not illuminate the entire circuit pattern on the reticle 201, but illuminates only a slit-like illumination area 206. On the semiconductor wafer 203, the image of the circuit pattern is transferred to only exposure area 207, which is a portion of semiconductor wafer 203. However, in this state of the circuit pattern, only the portion in the illumination area 206 is transferred. For this reason, the reticle 201 is scanned at a predetermined speed along a direction shown by arrow 208, and at the same time, the stage 204 is scanned along a direction shown by arrow 209 at a speed obtained by multiplying the scanning speed of the reticle 201 by the imaging magnification of the projection lens 202, to transfer the entire circuit pattern on the reticle 201 onto a shot area on the semiconductor wafer 203.
After the entire circuit pattern on the reticle 201 is transferred to the shot area of the semiconductor wafer 203, at a predetermined position, the stage 204 is moved (i.e., stepped) by a predetermined amount, and the circuit pattern is newly transferred to another shot area on the semiconductor wafer 203 by the same method. This procedure is repeated in the same manner as described above.
Referring to FIG. 10, reference numeral 210 indicates x-y-z coordinate axes. That is, an optical axis 211 of the projection lens 202 is defined in the z direction, the longitudinal directions of the illumination area 206 and the exposure area 207 are defined in the y direction, and the scanning directions of the reticle 201 and the stage 204 are defined in the x direction. This definition of the coordinate axes remains the same throughout this application, unless otherwise specified.
The reason why the step and scan exposure scheme can obtain an exposure area larger than that of a conventional stepper scheme, which is performed without using scanning, will be described below with reference to FIGS. 11(A) and 11(B).
In a conventional stepper scheme, an exposure area is limited to a range in which the aberration of a projection lens is preferably corrected. As an example, the range in which the aberration of the projection lens is corrected is indicated by a circle 221 (radius: r) in FIG. 11(A), and it is assumed that a circuit pattern is within a square area. In this case, the exposure area is the maximum square inscribed in the circle 221, i.e., a square whose side has a length of .sqroot.2r as indicated by region 222 in FIG. 11(A). The area 2r.sup.2 of the square is the area of an exposure region obtained by a conventional stepper scheme. In this case, the x and y axes of a coordinate system 223 are defined to have the same directions as those of two perpendicular sides of the square 222 as shown in FIG. 11(A).
As shown in FIG. 11(B), when the shape inscribed in the circle 221, whose aberration is corrected, is changed from a square to a rectangle 224, the length of the long side (y-axis direction) of the rectangle 224 becomes close to 2r. At this time, when the circuit pattern is scanned by a light flux in an area of the rectangle 224 in the x-axis direction to transfer the entire pattern onto a wafer, the area of the exposure area is determined to be 2rs (s: length in which scanning can be performed). This area can be set to be larger than the area 2r.sup.2. The step and scan scheme is designed for such an extension of the exposure area.
In a conventional step and scan exposure apparatus, both a reticle and a wafer are scanned to transfer an image. Therefore, synchronization between the reticle and the wafer must be precisely controlled during scanning.
However, when the scanning speed is increased to increase the throughput of the exposure apparatus, generation of a high-frequency asynchronous vibration component cannot be completely suppressed, and the positional relationship between the reticle and the wafer disadvantageously varies with time.
This phenomenon will be described below with reference to FIGS. 12(A) through 12(C). Referring to these figures, a coordinate system is defined as indicated by reference numeral 230 in FIG. 12(C), and it is assumed that a reticle 231 (FIG. 12(A)) and a wafer 232 (FIG. 12(C)) are arranged at optically conjugate positions with respect to the projection lens 233 (FIG. 12(B)). The reticle 231 is scanned in the x-axis positive direction (indicated by an arrow 234), and the wafer 232 is scanned in an x-axis negative direction (indicated by an arrow 235), thereby transferring the entire pattern.
In this case, attention is given to one small area (indicated by an area 236 in FIG. 12(A)) on the reticle 231. The moment the area 236 overlaps an illumination area 237 at time t=t1, the image of a pattern drawn at this portion is formed at the position of an image point 238 of the projection lens 233. In this case, the image point 238 is located at an end of an exposure area 239, and the image of the area 236 portion on the reticle 231 is transferred to the position of an area 240 on the wafer 232.
When scanning advances, the reticle 231 moves to positions indicated by time t=t2 and t=t3. (The reticle 231 is illustrated as being shifted in the z direction with time in FIG. 12(A) to make a positional change of the area 236 conspicuous. In actual scanning, however, the position of the area 236 is fixed in the z direction, and the reticle 231 and the wafer 232 move in only the x direction).
At time t=t2 and time t=t3, the images of patterns in the area 236 are formed at image points 241 and 242 of the projection lens 233, and the wafer 232 is controlled such that the position of the area 240 moves in accordance with these images. However, when the scanning speeds of the reticle 231 and the wafer 232 increase, the positions of the reticle 231 and the wafer 232 cannot be completely synchronized with each other. Consequently, as indicated by a shift amount .DELTA.x in FIG. 12(C), a position at which the image of the pattern is created is shifted from a position at which the image is supposed to be transferred.
The vibration component which cannot be completely removed by synchronous control is generally generated at a very high frequency, and the sign of the shift amount .DELTA.x is inverted many times while the area 240 passes through the exposure area 239. An image to be transferred to one position on the wafer 232 overlaps an image transferred to an adjacent position, since a variation in position of approximately .DELTA.x is repeated during scanning in the exposure area 239. As a result, an image in the scanning direction is degraded.
The above influence has been analyzed using simulation in, J. Bischoff, W. Henke, J. Werf, and P. Dirksen, "Simulation on Step & Scan Optical Lithography", Proceedings on SPIE, Vol. 2197, pp.953-964, 1994." An influence of a high-frequency asynchronous vibration component on the image of a pattern will be described below in accordance with the results analyzed in this paper.
As influences of vibration in scanning exposure, influences in the scanning direction (the x direction in FIGS. 12(A) through 12(C)) and in a direction (the y direction in these figures) perpendicular to the scanning direction may occur. Vibrations in both the directions are considered in this case. Assume that a positional shift amount .DELTA.y in the y direction is considered the same as a positional shift amount .DELTA.x in the x direction. In this case, it is known that variations in magnitude of the shift amounts .DELTA.x and .DELTA.y almost depend on the Gaussian distribution. As a probability density function related to position, a function expressed by the following equation can be defined for each of the x and y components: ##EQU1##
In this case, .sigma. is a standard deviation, where .sigma.=.sigma.x is set for .DELTA.=.DELTA.x, and .sigma.=.sigma.y is set for .DELTA.=.DELTA.y. Therefore, it is shown that an image having vibration can be obtained by calculating a function between a light intensity distribution obtained in an ideal state free from vibration and a function D(.DELTA.) given by the above equation.
A typical calculation described in the above paper is now discussed. FIGS. 13(A) and 13(B) show a result obtained such that an image of a 0.3.times.0.3 contact hole pattern is calculated under the conditions: wavelength of exposure light=248 nm; and NA (numerical aperture) of a projection optical system=0.5. FIG. 13(A) shows an ideal state free from vibration, and FIG. 13(B) shows a state wherein a high-frequency vibration having an amplitude of .sigma.y=50 nm exists in the y direction. In an actual exposure apparatus, vibration in the y direction can be suppressed to be less than the asynchronous component in the x direction (scanning direction). Therefore, the result shown in FIG. 13(B) is understood to be approximately a result obtained in a real system.
As is apparent from FIG. 13(B), the obtained intensity distribution has a shape extending in the scanning direction, and the pattern of the contact hole is transferred with distortion. This poses a serious problem when a semiconductor circuit element having a high integration density is manufactured.
FIGS. 14(A-1) to 14(B-3) are views for explaining results obtained by examining the influences of vibration on images of patterns each constituted by three lines. Each line width is 0.3 .mu.m, and the other conditions are the same as those in the above description. In this case, the influences of vibration on patterns are repeated in the x direction (scanning direction). FIG. 14(A-1) shows a contour line distribution of light intensity of an image free from vibration, and FIGS. 14(A-2) and 14(A-3) show contour line distributions of light intensities of images degraded when an asynchronous component increases. FIGS. 14(B-1) to 14(B-3) are obtained from FIGS. 14(A-1) to 14(A-3) by plotting light intensities along the ordinates.
Since vibration in the y direction is suppressed to be less than vibration in the x direction as described above, the image of a pattern repeated in the y direction is degraded less than the image of the pattern repeated in the x direction. As a result, when patterns having the same cycle/line width are considered, the characteristics of an image to be transferred are dependent on whether the pattern has a periodicity in the y direction (direction perpendicular to the scanning direction) or the x direction (scanning direction). For this reason, the size of a pattern on a resist obtained as a product varies depending on the above-referenced direction, and a serious hinderance occurs in the manufacture of semiconductor elements.